power: supply: document current direction

Currently the sign for CURRENT_NOW and CURRENT_AVG is a bit
of a mess. There are basically 3 different ways battery fuel
gauges report the current:

1. uses negative values for discharging and positive values
   for charging
2. uses positive values for discharging and negative values
   for discharging (opposit of 1)
3. only uses positive values

As a result userspace currently cannot use the sign at all in
a generic way. Let's solve the issue by documenting a canonical
way for reporting the data and ensure new drivers follow this
way. Then existing drivers can be fixed on a case-by-case basis.

The 'negative value = battery discharging' has been choosen,
since there are only very few drivers doing it the other way
around.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>